Ink-jet printers are finding increasing use due to their high printing speed, compactness and light weight. Such printers commonly employ as the ink an aqueous-based vehicle in which a dye is dissolved. The vehicle typically comprises water and a water-miscible organic compound, such as diethylene glycol (DEG), propylene glycol, or pyrrolidone.
The printers employ a print cartridge, which includes a reservoir for storing ink and a printhead for ejecting droplets of ink toward a print medium, such as paper. The apparatus for ejecting the ink droplets is often referred to as a drop generator.
The drop generator comprises an entrance channel (through which ink from the reservoir enters), a resistor surface, a border around the perimeter of the resistor defined by barrier walls, and a nozzle plate (nickel) with a nozzle above the resistor surface at the top of the barrier walls.
During the course of operation, two types of bubbles may be formed, one of which is undesirable. Undesirable bubbles usually are very small (about 0.1X resistor size) and stable; they may adhere continuously to the surface of the resistor or the interior surface of the nozzle plate or float loose within the drop generator. The ejection of ink is caused by an electrical pulse to the resistor, which creates the desirable vapor bubble, which then ejects ink through the nozzle and toward the print medium. The vapor bubble, or droplet, is unstable (of short time duration) and is as large as the resistor itself.
Ink bubbles stuck to the barrier wall or nozzle plate surface of the drop generator cause a reduction in drop ejection energy and stability, and, therefore, print quality is degraded. The bubbles on or very near the resistor surface cause low temperature nucleation with unstable, non-repeatable ejection droplet formation. The bubbles elsewhere in the drop generator absorb the pressure energy (the bubbles are compressible) and reduce ejection velocity, drop volume, and jet performance.
The geometry of the drop generator and the alignment of the nozzle plate are difficult to control in a manufacturing environment. In addition, controlling or modifying the resistor surface to obtain stable nucleation has not been successful to date. These factors, together with the high ink surface tension, result in degradation of the print quality.
Accordingly, a method is needed to improve the print quality.
The present invention teaches the addition of a propanol or ethanol or defoaming agent to the ink. Alcohols have been added to inks for various reasons; see, e.g., Eida et al, U.S. Pat. No. 4,295,889. However, such additions were made to inks having comparatively low water content. This is to be compared to the vehicles used in the inks discussed herein, which have at least about 90% water and less than about 10% of the water-miscible organic compound.